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Roots (Other Types of Roots and Root Modifications (Storage Roots (Roots…
Roots
Other Types of Roots and Root Modifications
Storage Roots
Roots frequently provide long-term storage for carbohydrates that accumulate during summer photosynthesis.
Aerial Roots of Orchids
Many orchids are epiphytic, living attached to the branches of trees. They are dangle free in the air. The orchids provide water in drought conditions.
Prop Roots
The stem of a monocot can become wider, with more vascular bundles, if it can produce adventitious roots that extend to the soil.
Contractile Roots
May be the means by which the shoot becomes anchored in the soil.
Mycorrhizae
Roots of most species of seed plants have a symbiotic relationship with soil fungi in which both organisms benefit.
Root Nodules and Nitrogen Fixation
Lack of nitrogenous compounds in the soil is a major growth-limiting factor. When some prokaryotes that can take in N2 die and decompose the roots are able to pick up and use the nitrogen in the soil.
Haustorial Roots of Parasitic Flowering Plants
Roots of parasitic plants have become highly modified and are known as haustoria.
Roots of Strangler Figs
When young, plants of strangler figs grow as epiphytes perched on a branch of a host tree.
Internal Structure of Roots
Zone of Maturation/Root Hair Zone
Root hairs grow outward, greatly increasing absorption of water and minerals.
The zone of elongation merges gradually with the zone of maturation, no distinct boundary exists because the differences between the two represent the gradual continued differentiation of the cells.
Although cortex cells continue to enlarge, their most significant activity is transfer of minerals from the epidermis to the vascular tissue.
Endodermis
Innermost layer of cortical cells differentiates into a cylinder
Casparian strips are involved in controlling the types of minerals that enter the xylem water stream.
Zone of Elongation
Just behind the root apical meristem itself is the region where cells expand greatly.
The zone of elongation is similar to the shoot's subapical meristem region.
The cells begin to differentiate into a visible pattern, although none of the cells are mature.
The outermost cells are protoderm and differentiate into epidermis.
In the center is the provascular tissue, cells that develop into primary xylem and primary phloem.
Mature Portions of the Root
Passage Cells are cells that that have only Casparian strips
The result of continued endodermis maturation is the formation of a watertight sheath around the vascular tissues.
The mature endodermis prevents root pressure which would force water to leak out into the cortex of older parts of the root instead of moving up into the shoot.
If water leaked into the stem cortex and filled intercellular spaces, oxygen diffusion would be prevented and the tissues would suffocate.
Root Apical Meristem
Regular files of cells can be seen to originate in the meristem and extend into the regions of mature root tissues.
The root is more orderly than the shoot because it experiences no disruptions because of leaf primordia, leaf traces, or axillary buds.
If part of the root apical meristem is damaged, quiescent center cells become active and form a new apical meristem.
After the new meristem is established, its central cells become inactive, forming a new quiescent center.
Root Cap
The structure and growth
As cells are pushed forward they develop dense starch grains and their endoplasmic reticulum becomes displaced to the forward end of the cell.
As cells are pushed closer to the edge of the cap their structure and metabolism change dramatically.
Endoplasmic reticulum becomes less conspicuous starch grains are digested, and the cell's dictyosomes secrete copious amounts of mucigel by exocytosis.
It's estimated that only 4 or 5 days pass form cell formation in the root cap to its sloughing off.
External Structure of Roots
Organization of Root Systems
Plants have a highly branched root system
Most seed plants have a single prominent taproot that is much larger than all the rest and numerous small lateral roots or branch roots coming out of it.
This taproot develops from the embryonic root, or radicle, that was present in the seed.
After germination it grows and usually becomes the largest root in the system.
Beets
All have fine taproots, but they are removed before being sold.
Turnips
Carrots
Most monocots and some eudicots have a mass of many similarly sized roots constituting a fibrous root system.
Happens after radicle dies during or right after germination.
Roots that are initiated in the stem tissue, but do not arise on pre-existing roots and are not radicals, they're known as adventitious roots.
Adventitious roots increase absorptive and transport capabilities of the root system.
Structure of Individual Roots
Individual roots are simple, with no leaves or leaf scars and no leaf axils or axillary buds.
The root tip is the region where growth in length occurs.
In roots growth by discrete apical meristems is the only feasible type of longitudinal growth.
In animals everything grows simultaneously, whereas in roots and stems it only grows at small meristematic regions.
The root is embedded in a solid matrix, it's impossible for it all to grow at once.
During apical growth, only the extreme tip pushes through soil.
The root cap is a thick layer of cells protecting the root apical meristem.
Mucigel is a complex polysaccharide secreted by dictyosomes of root cap cells, it lubricates the passage of the root through the soil.
Mucigel is rich in carbohydrates and amino acids which help rapid growth of soil bacteria around the root tip.
Just behind the zone of elongation is the root hair zone, a region where many of the epidermal cells extend out as narrow trichomes.
Origin and Development of Lateral Roots
Lateral roots are initiated by cell divisions in the pericycle.
As the root primordium swells into the cortex, the endodermis may be torn or crushed or may undergo cell division and form a thin covering over the primordium.
As it pushes outward, the new lateral root destroys cells of the cortex and epidermis that lie in its path, ultimately breaking the endodermis.
Lateral roots are initiated deep within the root
